Microwave filter comprising a coaxial structure

ABSTRACT

The invention relates to a microwave filter comprising a coaxial structure, consisting of a tubular outer conductor and an inner bar conductor. According to the invention, the inner bar conductor extends in an axial direction inside the outer tube and, together with said tube, forms a series of concentric slots in the axial direction thereby defining successive coaxial line segments with low characteristic impedance and coaxial line segments with high characteristic impedance. The aforementioned concentric slots are produced in a synthetic foam block.

The invention relates to a coaxial structure microwave filter comprisingan outer conductive core and an inner conductive core extendingaccording to an axial direction within the outer core and forming withthis core a succession of concentric crenelations according to an axialdirection defining successive sections of low characteristic impedancecoaxial lines and high characteristic impedance coaxial lines.

The work “Microwave Filters, Impedance-Matching Networks and CouplingStructures”, MgrawHill, 1962, describes such a microwave filter, inparticular a low-pass filter, in which the outer conductive core isnormally constituted by a cylindrical metal rod carrying concentricmetal disks spaced according to the axial direction, the metal disksforming the succession of concentric crenelations. The cross-section ofthe inner core thus varies according to the axial direction so that eachsection of the large diameter inner core (corresponding to a metal disk)defines a section of coaxial line of very low impedance and each sectionof inner core of smaller diameter (corresponding to the interval betweentwo consecutive disks) defines a section of coaxial line of highimpedance. The dimensions of the sections are adjusted so as to realizethe transfer function of the filter. However, the realisation of such acoaxial structure microwave filter proves to be complex and costly,particularly for maintaining a perfect coaxiality between the inner coreand the outer core of the filter. Spacers made from plastic or anotherdielectric material are generally used to maintain the coaxiality butthis introduces dielectric losses.

The invention proposes a coaxial structure microwave filter of a simplerand less expensive construction suitable for low cost volume production.

For this purpose, the invention relates to a coaxial structure microwavefilter constituted by a tube of synthetic foam material, the tubepresenting a constant internal diameter and a fully metallized externalsurface with, in the axial direction, a profile according to a periodicor constant function and by a bar of a fully metallized syntheticmaterial, with a constant external profile or following a periodicfunction, the largest diameter of the bar being noticeably equal to theinternal diameter of the tube so that the bar can be inserted into thetube while maintaining the coaxiality between the tube and the bar. Thefoam used is preferably a polymethacrylimide foam known for itselectrical characteristics approaching those of air, for its mechanicalcharacteristics of rigidity and lightness and for its low cost price. Inparticular, a polymethacrylimide foam commercialised under the name of“ROHACELL HF” can be used.

According to the particularities of a filter according to the invention:

-   -   The periodic or constant function per part depends on        crenelations, the crenelations being able to have dimensions        that differ from one crenelation to another.    -   The thickness of the tube is chosen to maintain electrical        insulation between the metallized surface of the tube and the        bar

With this construction, a microwave filter can easily be combined with amonopole type or dipole type antenna.

The invention extends to a method of producing a microwave filter asdefined above according to which the periodic function is realized bythermoforming the foam tube or foam bar. In particular, as athermoforming technique, hot press moulding will preferably be used,which is adapted to an objective of high volume, low cost production.

The metallization of the foam tube or foam bar is preferably anon-directive metallization by projection or brush.

Embodiments of a filter according to the invention are described belowand illustrated in the drawings.

FIG. 1 shows an exploded perspective, in a highly schematic manner, of afirst embodiment of a coaxial structure microwave filter according tothe invention.

FIG. 2 schematically shows an axial section of a second embodiment of acoaxial structure microwave filter according to the invention associatedwith a monopole type antenna.

FIG. 3 schematically shows an axial section of a filter according to thefirst embodiment associated with a dipole type antenna.

A first example of a coaxial structure microwave filter according to theinvention is shown in FIG. 1 according to an exploded perspective view.

The outer conductive tube 1 and the inner conductive bar 2 of the filterare shown in FIG. 1 dissociated from each other for greater clarity, butit must be understood that the inner bar 2 extends according to theaxial direction A inside the outer tube 1.

The inner bar 2 of the filter is constituted by a cylindrical bar madeof synthetic foam whose outer surface follows a periodic functionaccording to the axial direction. It preferably forms a succession ofconcentric crenelations 3A to 3D realizing the transfer function of thefilter, for example a transfer function of a low-pass filter by definingsuccessive sections of low characteristic impedance coaxial lines andhigh characteristic impedance coaxial lines. The conformation of thefoam bar 2 is realized by thermoforming, in particular according to ahot press moulding technique. The outer surface of foam bar 2 ismetallized preferably by projection or by brush.

The outer tube 1 of the filter is constituted by a cylindrical tube ofsynthetic foam having a constant inner cross-section, the inner diameterof the tube being very slightly greater at the largest outer diameter ofthe foam bar 2 to allow the bar to be inserted into the tube. Thecylindrical tube 1 has an outer surface fully metallized according tothe technique described above. The thickness of the tube 1 is chosen torealize an electrical insulation between its outer metallized surfaceand the bar

The synthetic material foam used is preferably a polymethacrylate imidefoam.

The structure of the filter shown in FIG. 1 can be reinforced by twohalf-shells (not shown) surrounding the tube 1 that can be realized in aplastic material or in synthetic foam material.

Naturally, the tube 1 and foam bar 2 can have a cross-section other thancircular, for example rectangular or square without falling outside thescope of the invention.

FIG. 2 shows another embodiment of a filter according to the invention.The outer tube 1′ of the filter is constituted by a constituted by acylindrical tube of synthetic foam material whose outer metallizedsurface is conformed to define the succession of crenelations 3A′-3B′according to the axial direction A whereas the inner bar 2′ of thefilter is constituted by a conductive cylindrical bar of constantcross-section. In this manner, the outer surface of the tube presents,according to the axial direction, a profile following a periodic orconstant function by parts such as a crenelation function. Theconductive bar 2′ can consist of a solid or hollow cylindrical metaltube. The bar 2′ can also be constituted by metallized syntheticmaterial foam. In FIG. 2, the microwave filter according to theinvention is associated with a monopole type antenna 4 constituted by anextension of the inner core 2′ of the filter.

FIG. 3 shows a microwave filter according to the invention that issimilar to the filter shown in FIG. 1 with an outer foam tube 1″ ofconstant cross-section and an inner bar constituted by a foam bar 2″ ofvariable cross-section according to the axial direction A. Here, thefilter is associated with a dipole type antenna 5.

The use of the metallized foam technique enables complex coaxialstructure microwave filters to be realized at low cost.

1. A coaxial structure microwave filter comprising a tube presenting aconstant inner diameter and a fully metallized outer surface with, inthe axial direction, a profile according to a periodic or constantfunction and an inner bar with a fully metallized outer profile Offollowing a constant or periodic function, tube and the bar beingrealized in foam of a metallizable material, the largest diameter of thebar being noticeably equal to the inner diameter of the tube.
 2. Thefilter according to claim 1, wherein the periodic function is acrenelation function, the crenelations having dimensions identical fromone crenelation to another.
 3. Process for manufacturing, coaxialstructure microwave comprising a tube presenting a constant innerdiameter and a fully metallized outer surface with, in the axialdirection, a profile according to a periodic or constant function and aninner bar with a fully metallized outer profile following a constant orperiodic function, the tube and the bar being realized in foam of ametallizable material, the largest diameter of the bar being noticeablyequal to the inner diameter of the tube in which the periodic functionis realized by thermoforming the foam tube or foam bar.
 4. The processof manufacturing according to claim 4, in which the foam tube or foambar is metallized at the surface by projection or by brush.
 5. Thefilter according to claim 1, wherein the periodic function is acrenelation function, the crenelations having dimensions different fromone crenelation to another.